Rumen bacteria elaborate an enzyme, urease. Urease
is capable of breaking down urea to ammonia and carbon
dioxide. Rumen bacteria then incorporate the ammonia
into new amino acids and bacterial protein. Thus, urea
can be used as a non-nitrogen source for ruminants.
Unfortunately, urease often makes ammonia available
faster than it can be used by rumen bacteria. That leads
to poor utilization of urea or, in extreme cases, to
toxicity.